If one tunes an HF/MF radio at night, the lower the frequency (or the longer the wavelength) of the band, the more QRN (static and atmospheric noise) there is. 80m is noisier than 40m, and 160m is noisier than 80m. (I haven't operated on the exotic new 630m or 2200m bands yet, but I wouldn't be surprised to hear that they are even worse.)
Why is the QRN worse the lower one tunes in frequency on the HF and MF bands?
The primary source of atmospheric radio noise is the estimated 40 lightning strikes that occur each second around the world. NASA's Review of Measurements of the RF Spectrum of Radiation from Lightning (1986) shows a 1/f decline in electromagnetic field strength vs. frequency from lightning discharge:
Electromagnetic energy radiated from these strikes at the varying frequencies propagates to different distances via the usual means of reflection and refraction from the earth and the ionosphere.
